As MOSFET devices are scaled down smaller and smaller in ultra large scale integration (ULSI) processes, all aspects of the MOSFET device structures, including semiconductor channels, source and drain electrical contacts, gate stacks, and interconnects, need to be scaled. Increasingly, the now famous Moore's law has also become applicable to novel materials. Very prominent among these novel materials are metal silicides. Metal silicides are already of paramount importance as integral parts of silicon microelectronics, mainly because of the low resistivity ohmic contact to CMOS transistors provided by metallic silicides such as NiSi, COSi2, and TiSi2 in the now dominant SALICIDE (Self-Aligned Silicides) process. In the last few years, metallic silicides, particularly nickel silicides (NiSi, Ni2Si, Ni3Si) have also been successfully explored in the FUSI (Full Silicidated Gate) process for the state-of-art MOSFET at 45 nm node, which is superior to the traditionally poly-silicon gates. Often overlooked is the fact that many silicides are direct bandgap semiconductors (e.g., β-FeSi2, CrSi2) that are promising for photonic applications as well, though a host of materials issues have prevented their application so far. Most significantly, the recent discovery of group IV ferromagnetic semiconductors, such as alloys of FexCo1-x,Si, suddenly brings into sight the exciting prospects of silicon-based spintronics. Furthermore, because silicides are omnipresent at the interfaces between semiconductor (e.g., Si) and ferromagnetic metals (such as Fe), silicides inevitably dictate the success or failure of any silicon-based spintronic devices. Therefore, silicide materials are critically important both by choice and by necessity.
One the other hand, the tremendous progress witnessed over the last decade in one-dimensional nanomaterials such as carbon nanotubes and semiconductor nanowires has enabled a bottom-up paradigm of nanoscience and nanotechnology in which any kind of devices or functional systems can be assembled from the bottom-up using chemically synthesized nanoscale building blocks. Semiconductor nanowires have enjoyed prominent success, not only because these materials can exhibit diverse physical behavior and simultaneously function as the wires that access and interconnect devices, but also because they can be synthesized in single-crystalline form with precisely controlled (hetero)structures, diameters, lengths, chemical compositions and doping/electronic properties using a nanocluster catalyzed vapor-liquid-solid (VLS) growth process. Such high quality nanoscale building blocks have enabled the bottom-up assembly of many integrated electronic and photonic devices, including high performance nanometer scale field-effect transistors (FETs), light-emitting diodes, photodetectors, and lasers.
To date, there are few reports of the production of any silicides nanowires. Of these few reports, one report describes NiSi nanowires converted from silicon nanowires by reaction with an evaporated nickel film. See, Wu, et al., Nature, 430, 61 (2004). Other reports describe self-assembled epitaxial nanowires of rare earth silicides on vicinal silicon surfaces. See, Chen, et al., Appl. Phys. Leu., 76, 4004 (2000); and He et al., Phys. Rev. Lett., 93 (2004). The lack of reports is likely due to the fact that a general and rational chemical synthesis of these nanomaterials is challenging due to the complex phase behavior of silicides. For instance, there are five known iron silicide compound phases (Fe2Si, Fe5Si3, FeSi, α-FeSi2, and (β-FeSi2) and four known chromium silicide compound phases (Cr3Si, Cr5Si3, CrSi, CrSi2).
Rational synthesis of nanowires usually has two main challenges: the anisotropic crystal growth to form 1-D nanostructures and the delivery of source materials. The most successful technique for metal nanowire growth may be the metal-catalyzed vapor-liquid-solid (VLS) technique which uses metallic nanoparticles that form low temperature eutectics with the nanowire material via chemical vapor deposition (CVD). Unfortunately, this technique is not suited for growing a wide range of metal nanowires and produces nanowires that are contaminated with metal catalyst impurities and catalyst “tips”